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The oldest I have transferred is my Gardevoir from Emerald. Oldest I have overall is my Emerald Swampert, but as I was in the process of transferring shit from Pearl to White for banking last night, I found out they still have a restriction on HMs, and Swampert probably knows surf or waterfall, so I need to go take care of that. Pain in the ass to have to remove HMs, honestly.
I wish there was some way to transfer from the GSC though, I really do. A few weeks ago I decided to go see if the save battery in my copy of Crystal was dead. It's not. My first Pokemon ever, the Feraligatr I picked as my starter in Gold, is still there. Silly child I was, she's level 100 with Scratch, Surf, Whirlpool, and Waterfall. I feel like I should get a new battery for Gold so I can transfer her back, among others. Crystal will probably give out within the next year (I'm amazed it hasn't already), and everything I had as a kid will be gone then.